Sand the wood smooth and remove all dust
Seal all edges, corners, and end grain first
Apply a waterproof wood sealer
Use exterior-grade polyurethane
Use spar varnish for outdoor wood
Apply marine varnish for maximum water resistance
Use penetrating epoxy for deep protection
Use wood oil such as teak oil or tung oil
Apply multiple thin coats
Let each coat dry fully before the next
Recoat regularly as needed
Caulk joints and seams
Keep wood elevated and away from standing water
Paint the wood with exterior waterproof paint
Use a primer before painting
Store wood in a dry, covered area